The present invention relates to solenoid operated control valves for hydraulic systems, and more particularly to such valves of a force-feedback type.
Construction and agricultural equipment have moveable members which are operated by a hydraulic cylinder and piston combination. The cylinder is divided into two internal chambers by the piston and alternate application of hydraulic fluid under pressure to each chamber moves the piston in opposite directions.
Application of hydraulic fluid to the cylinder typically is controlled by a manually operated valve, such as the one described in U.S. Pat. No. 5,579,642. In this type of valve, a manual lever was mechanically connected to a spool within a bore of the valve. A human equipment operator moves the lever to place the spool into various positions with respect to cavities in the bore that communicate with a pump outlet, a fluid reservoir or the cylinder. Moving the spool in one direction controls the flow of pressurized hydraulic fluid from the pump to one of the cylinder chambers and the fluid flow from the other chamber to the reservoir. Moving the spool in the opposite direction reverses the application and draining of fluid with respect to the cylinder chambers. By varying the degree to which the spool is moved in the appropriate direction, the rate at which fluid flows into the associated cylinder chamber can be varied, thereby moving the piston at proportionally different speeds.
In addition, some control valves provide a "float" position in which both cylinder chambers are connected simultaneously via the spool to the fluid reservoir. This position allows the member driven by the cylinder to move freely in response to external forces. For example, a snow plow blade may be allowed to float against the pavement to accommodate variations in surface contour and avoid digging into the pavement.
There is a trend with respect to construction and agricultural equipment away from manually operated hydraulic valves toward electrically controlled solenoid valves. This type of system simplifies the hydraulic plumbing as the control valves can be located near the cylinder and not in the operator cab. This change in technology also facilitates computerized regulation of various machine functions.
Solenoid valves are well known for controlling the flow of hydraulic fluid and employ an electromagnetic coil which moves an armature in one direction to open a valve. Either the armature or a valve member is spring loaded to close the valve when electric current is removed from the coil.
In order to actuate a standard bidirectional spool valve with a solenoid mechanism, a separate solenoid actuator had to be connected to each end of the spool. This significantly increased the overall length of the valve assembly which was disadvantageous in some installations. In addition, this configuration requires a control circuit that prevents both solenoid actuators from being energized simultaneously and working against each other.
As an alternative, hydraulic systems have been devised which utilize a pair of solenoid valves for each cylinder chamber to be power driven. For a given cylinder chamber, one solenoid valve controls the application of fluid under pressure from a pump to move the piston in one direction, and the other solenoid valve is alternatively opened to drain fluid from the given chamber to a tank to move the piston in the opposite direction. If both chambers of a cylinder are to be power driven, four such solenoid valves are required, two supply valves and two drain valves.